1. Field of the Invention
This invention relates to magnetic sensors using magnetoresistive elements such as giant magnetoresistive (GMR) elements. This invention also relates to manufacturing methods for manufacturing magnetic sensors.
This application claims priority on Japanese Patent Application No. 2002-304392 and Japanese Patent Application No. 2003-65200, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, various types of magnetic sensors using magnetoresistive elements such as giant magnetoresistive (GMR) elements have been developed and reduced to practice.
A typical example of a GMR element comprises a pinned layer in which magnetization is pinned in a prescribed direction, and a free layer whose magnetization direction varies in response to an external magnetic field. That is, when an external magnetic field is applied, the GMR element presents resistance in response to a relative relationship in magnetization direction between the pinned layer and free layer; therefore, it is possible to detect the external magnetic field by measuring the resistance of the GMR element.
In order to detect minor external magnetic fields at a high accuracy, it is necessary for the aforementioned magnetic sensor to stably maintain the magnetization direction of each of the magnetized sections of the free layer to match a prescribed direction (hereinafter, referred to as an initialization direction) under the condition where no external magnetic field is applied to the magnetic sensor.
In general, a thin-film free layer is formed in a rectangular shape in plan view, so that a long side (e.g., a long axis or a longitudinal direction) of the rectangular shape is directed to match the aforementioned initialization direction so as to establish shape anisotropy in which the magnetization directions are aligned to match the longitudinal direction. By using shape anisotropy, the magnetization directions of the magnetized sections of the free layer are aligned to match the initialization direction. In order to stably restore and maintain the magnetization directions of the magnetized sections of the free layer in the initialization direction over a long term after an external magnetic field disappears, bias magnet films corresponding to permanent magnets are arranged at both ends of the free layer in the longitudinal direction, so that a prescribed magnetic field of the initialization direction is applied to the free layer by the bias magnet films.
In magnetoresistance-effect elements (i.e., magnetoresistive elements) of an AMR type, it is necessary to apply bias magnetic fields in order to increase sensitivities. In order to uniformly apply a bias magnetic field to four magnetoresistive elements, for example, they are inclined relative to a substrate by a prescribed angle of 45°. An example of a magnetic sensor in which magnetoresistive elements are inclined relative to a substrate is disclosed in Japanese Patent Application Publication No. Hei 5-126577 (see paragraph [0016], and FIG. 5(a)).
When an external magnetic field, which is relatively large and less than the coercive force of a bias magnet film and whose magnetization direction is opposite to the initialization direction, is applied to the conventionally-known magnetic sensor, each of the magnetized sections of the free layer is changed in magnetization direction; thereafter, when the external magnetic field disappears, each of the magnetized sections of the free layer cannot be restored and may not match the initialization direction. This deteriorates the detection accuracy of the magnetic sensor for sensing a magnetic field applied thereto.
It is very difficult to form two or more magnetoresistive elements, in which the magnetization directions of the pinned layers mutually cross each other, on a small substrate; therefore, no single chip having such a configuration has been developed and produced. That is, the conventionally-known magnetic sensor cannot be reduced in size, and it is very difficult to broaden an application range therefor due to a restriction regarding the magnetization direction of the pinned layer.
To cope with the aforementioned situation, it is possible to develop a two-axis magnetic sensor, using GMR elements, that can be reduced in size and that can be broadened in the application range, which is disclosed in Japanese Patent Application No 2001-281703.
FIG. 26 is a plan view showing a two-axis magnetic sensor using GMR elements, wherein a magnetic sensor 101 comprises a quartz substrate 102 having a roughly square shape and a prescribed thickness as well as X-axis GMR elements 111 to 114, and Y-axis GMR elements 121 to 124. Herein, all of the X-axis GMR elements 111-114 are formed on the quartz substrate 102 and are combined together to form an X-axis magnetic sensor for detecting magnetic fields in the X-axis direction, and all the Y-axis GMR elements 121-124 are formed on the quartz substrate 102 and are combined together to form a Y-axis magnetic sensor for detecting magnetic fields in the Y-axis direction perpendicular to the X-axis direction.
Two pairs of the X-axis GMR elements 111-112 and 113-114 are respectively arranged in proximity to the midpoints on two sides of the quartz substrate 102, which cross at a right angle to the X-axis, in such a way that they are arranged in parallel with each other. Similarly, two pairs of the Y-axis GMR elements 121-122 and 123-124 are respectively arranged in proximity to the midpoints on two sides of the quartz substrate 102, which cross at a right angle to the Y-axis, in such a way that they are arranged in parallel with each other.
The X-axis GMR elements 111 to 114 and the Y-axis GMR elements 121 to 124 differ from each other in their arrangements on the quartz substrate 102 and in their magnetization directions pinned in the pinned layers thereof. With the exception of these points, they are formed in the same configuration.
Therefore, the X-axis GMR element 111 is taken as an example whose configuration is to be described below.
As shown in FIGS. 27 and 28, the X-axis GMR element 111 comprises band-shaped spin valve films 131, which are arranged in parallel with each other, and bias magnet films 132, each of which corresponds to a thin film of a hard ferromagnetic substance, composed of CoCrPt and the like, having a high coercive force and a high squareness ratio.
The spin valve films 131 are respectively paired and connected together via the bias magnet films 132 at both ends thereof in such a way that one bias magnet film is arranged at one end of the ‘paired’ spin valve films, and the other bias magnetic film is arranged at the other end of the ‘adjacent paired’ spin valve films. In short, the spin valve films 131 are connected together via the bias magnet films 132 in a zigzag manner.
As shown in FIG. 29, the spin valve film 131 is formed in a sequential lamination of various layers on the quartz substrate 102, namely: a free layer F; a conductive spacer layer S, composed of Cu, having a film thickness of 2.4 nm (or 24 Å); a pinned layer PD composed of CoFe; a pinning layer PN composed of PtMn; and a capping layer C made of a thin metal film composed of titanium (Ti), tantalum (Ta), and the like.
The free layer F is changed in magnetization direction in response to the direction of an external magnetic field applied thereto, and it is formed by a CoZrNb amorphous magnetic layer 131a having a film thickness of 8 nm (or 80 Å), a NiFe magnetic layer 131b having a film thickness of 3.3 nm (or 33 Å) that is laminated on the CoZrNb amorphous magnetic layer 131a, and a CoFe layer 131c whose film thickness approximately ranges from 1 nm to 3 nm (or 10 Å to 30 Å) that is laminated on the NiFe magnetic layer 131b. 
In order to maintain single-axis anisotropy of the free layer F, a bias magnetic field is applied to the free layer F by the bias magnet film 132 in the Y-axis direction shown in FIG. 27.
The spacer layer S is a thin metal film composed of Cu or a Cu alloy.
Both of the CoZrNb amorphous magnetic layer 131a and the NiFe magnetic layer 131b are formed from soft ferromagnetic substances. In addition, the CoFe layer 131c blocks Ni diffusion of the NiFe magnetic layer 131b and Cu diffusion of the spacer layer S.
The pinned layer PD is formed by a CoFe magnetic layer 131d having a film thickness of 2.2 nm (or 22 Å). The CoFe magnetic layer 131d is backed by an antiferromagnetic film 131e, which will be described later, in a switched connection manner so that the magnetization direction thereof is subjected to pinning (or anchoring) in the negative direction of the X-axis.
The pinning layer PN is formed by the antiferromagnetic film 131e having a film thickness of 24 nm (or 240 Å) laminated on the CoFe magnetic layer 131d, wherein the antiferromagnetic film 131e is composed of a PtNm alloy including Pt at 45-55 mol %. When a magnetic field is applied in the negative direction of the X-axis, the antiferromagnetic film 131e is changed to an ordered lattice.
Hereinafter, the combination of the pinned layer PD and the pinning layer PN will be generally called a pin layer.
All of the other X-axis GMR elements 112-114 and the Y-axis GMR elements 121-124 have the same configuration as the X-axis GMR element 111 described above; hence, the detailed descriptions thereof will be omitted.
Next, a description will be given with respect to the magnetic properties (or magnetic characteristics) of the X-axis GMR elements 111-114 and the Y-axis GMR elements 121-124.
FIG. 30 shows a graph regarding variations of resistance relative to the magnitude of an external magnetic field applied to the X-axis GMR element 111. Herein, ‘solid’ curves represent hysteresis characteristics relative to variations of the external magnetic field in the X-axis, in which the resistance varies approximately proportional to the external magnetic field in a prescribed range between −Hk and +Hk, but the resistance is maintained substantially constant in both of the other ranges outside of the prescribed range. In addition, ‘dotted’ curves represent characteristics relative to variations of the external magnetic field in the Y-axis, in which the resistance is maintained substantially constant.
In FIG. 26, magnetization directions of pinned layers adapted to the X-axis GMR elements 111-114 and the Y-axis GMR elements 121-124 are shown by arrows, which are directed opposite to each other.
That is, both of the X-axis GMR elements 111 and 112 have the same magnetization direction of the pinned layer that is pinned by the pinning layer along the negative direction of the X-axis.
Both of the X-axis GMR elements 113 and 114 have the same magnetization direction of the pinned layer that is pinned by the pinning layer along the positive direction of the X-axis.
In addition, both of the Y-axis GMR elements 121 and 122 have the same magnetization direction of the pinned layer that is pinned by the pinning layer along the positive direction of the Y-axis.
Both of the Y-axis GMR elements 123 and 124 have the same magnetization direction of the pinned layer that is pinned by the pinning layer along the negative direction of the Y-axis.
The aforementioned X-axis magnetic sensor is constituted by arranging the X-axis GMR elements 111-114 in a full bridge connection as shown in FIG. 31, wherein arrows accompanied with blocks show magnetization directions of pinned layers pinned by pinning layers. In the aforementioned constitution, a dc power source is used to apply voltage Vxin+ (e.g., 5 V) at one terminal and to apply voltage Vxin− (e.g., 0 V) at the other terminal, whereby Vxout+ appears at a terminal H that is derived from the connection between the X-axis GMR elements 111 and 113, and Vxout− appears at a terminal L that is derived from the connection between the X-axis GMR elements 112 and 114. Herein, it is possible to extract a potential difference (or a voltage difference) (Vxout+−Vxout−) as an output voltage Vxout.
In short, the X-axis magnetic sensor presents characteristics relative to variations of an external magnetic field in the X-axis, in which, as shown by the solid curves in FIG. 32, the output voltage Vxout thereof is changed substantially proportional to the external magnetic field in a prescribed range between −Hk and +Hk, and it is maintained substantially constant in other ranges outside of the prescribed range.
In addition, the output voltage Vout is substantially maintained at 0 V relative to variations of the external magnetic field in the Y-axis, which is shown by the dotted curves in FIG. 32.
Similar to the aforementioned X-axis magnetic sensor, the Y-axis magnetic sensor is constituted by arranging the Y-axis GMR elements 121-124 in a full bridge connection as shown in FIG. 33. In this constitution, a dc power source is used to apply voltage Vyin+ (e.g., 5 V) at one terminal and to apply voltage Vyin− (e.g., 0 V) at the other terminal, whereby Vyout+ appears at a terminal H that is derived from the connection between the Y-axis GMR elements 122 and 124, and Vyout− appears at a terminal L that is derived from the connection between the Y-axis GMR elements 121 and 123. Herein, it is possible to extract a potential difference (Vyout+−Vyout−) as an output voltage Vyout.
In short, the Y-axis magnetic sensor presents hysteresis characteristics relative to variations of an external magnetic field in the Y-axis, in which, as shown by dotted curves in FIG. 34, the output voltage Vyout thereof is changed substantially proportional to the external magnetic field in a prescribed range −Hk and +Hk, and it is maintained substantially constant in other ranges outside of the prescribed range.
In addition, the output voltage Vyout is substantially maintained at 0 V relative to variations of the external magnetic field in the Y-axis, which is shown by the solid curves in FIG. 34.
Next, a description will be given regarding a manufacturing method of the magnetic sensor 101.
As shown in FIG. 35, a plurality of island-like regions, corresponding to films M which contribute to formation of individual GMR elements, are formed on the surface of a quartz glass 141 having a rectangular shape. When the quartz glass 141 is subjected to a cutting process along break lines B and is thus divided into individual quartz substrates 102, the films M are arranged at prescribed positions to match the X-axis GMR elements 111-114 and the Y-axis GMR elements 121-124. In addition, alignment marks (i.e., positioning marks) 142 are formed on four corners of the quartz glass 141, wherein each of them is formed in a roughly rectangular shape from which a cross-shaped region is removed.
Next, there are provided a plurality of rectangular metal plates 144, each of which, as shown in FIGS. 36 and 37, has a plurality of through holes 143 having square openings, which are formed and regularly arranged in a lattice-like manner. In addition, permanent magnets 145, each having a rectangular parallelopiped shape whose cross-sectional shape substantially matches the opening shape of each of the through holes 143, are respectively inserted into the through holes 143 in such a way that the upper end surfaces of the permanent magnets 145 respectively inserted into the through holes 143 are all arranged in the same plane substantially in parallel with the surface of the metal plate 144, wherein the ‘adjacent’ permanent magnets 145 differ from each other in polarity.
Next, there is provided a plate 151, which is shown in FIG. 38, made of a transparent quartz glass having substantially the same shape as the metal plate 144. In addition, cross-shaped alignment marks (or positioning marks) 152 are formed on the four corners of the plate 151 to cooperate with the aforementioned alignment marks 142 of the quartz glass 141, thus establishing positioning between the quartz glass 141 and the plate 151. In addition, a plurality of alignment marks 153, each of which matches the contour shape of each of the permanent magnets 145, are formed in conformity with the positions of the through holes 143 of the metal plate 144.
The upper end surfaces of the permanent magnets 145 are adhered to the lower surface of the plate 151 by use of a prescribed adhesive. At this time, a prescribed positioning is established between the metal plate 144 (holding the permanent magnets 145) and the plate 151 by use of the alignment marks 153.
Thereafter, the metal plate 144 is removed from the lower side of the plate 151. Thus, it is possible to produce a magnet array in which the permanent magnet 145 are arranged on the plate 151 in a lattice-like manner and in which the ‘adjacent’ permanent magnets differ from each other in polarity.
As shown in FIG. 40, the quartz glass 141 is brought into contact with the plate 151 in such a way that the aforementioned films M come into contact with the upper surface of the plate 151. Herein, the prescribed positioning is established between the quartz glass 141 and the plate 151 by mutually matching the alignment marks 142 with the alignment marks 152. Then, fixing members 155 such as clips are used to fix the quartz glass 141 and the plate 151 together.
In the aforementioned state, as shown in FIG. 41, magnetic forces are formed in directions from the N pole of one permanent magnet 145 towards the S poles of adjacent permanent magnets 145. Therefore, as shown in FIG. 42, magnetic forces are applied to the films M, which are arranged to encompass one permanent magnet 145, in four directions, that is, the positive direction of the Y-axis, the positive direction of the X-axis, the negative direction of the Y-axis, and the negative direction of the X-axis.
The quartz glass 141 and the plate 151 fixed together by the fixing members 155 are subjected to a heat treatment for four hours at a prescribed temperature ranging from 250° C. to 280° C., for example. Thus, it is possible to order the pinning layers and to pin the pinned layers of the GMR elements. The quartz glass 141 and the plate 151 are separated from each other, and passivation films and polyimide films are formed for the purpose of protection; then, the quartz glass 141 is subjected to cutting on break lines B. Thus, the magnetic sensor 101 is produced.
Compared with the conventionally-known magnetic sensor in which magnetoresistive elements of the AMR type are inclined at 45° relative to the substrate, the aforementioned two-axis magnetic sensor has an advantage which allows magnetic measurement on geomagnetic levels without using bias magnetic fields; however, when applied with an intense magnetic field, the magnetized states thereof are unexpectedly changed so as to cause unwanted offsets in the bridge configurations of the GMR elements.
To cope with the aforementioned drawback, it is possible to suppress offset variations against the influence of an intense magnetic field by attaching permanent magnets to both ends of the GMR elements. Specifically, a relatively great magnetic field that is greater than the coercive force Hc of the permanent magnet is applied to the GMR element in the longitudinal direction, i.e., longitudinal direction of the free layer, so that the free layer is being initialized at the same time that the permanent magnet is attached so as to cause magnetization. Herein, it is possible to use the aforementioned magnet array, which is used in an ordering heat treatment of pin layers, in this method.
In the aforementioned method, however, it is necessary to apply a magnetic field in a direction perpendicular to the longitudinal direction of the GMR element in the ordering heat treatment, and it is also necessary to apply a magnetic field whose magnetism is identical to that of the permanent magnet in the longitudinal direction of the GMR element. Herein, magnetic fields of different directions are required in the aforementioned steps.
In the magnet array, under the ordering heat treatment, each of the permanent magnets should be arranged such that the center of gravity thereof is coincident with the center of each cell on the quartz glass, and when each of them is arranged to cause magnetization, it should be shifted in position so that the center of gravity thereof is coincident with each of the four corners of the quartz glass. This may cause positional deviations, which in turn cause the initialization direction to be shifted and thus deteriorates the measurement accuracy. When the aforementioned magnetic sensor is used under the influence of an intense magnetic field, offsets become easy to vary.
As described above, the aforementioned two-axis magnetic field may have an advantage in the reduction of the hysteresis characteristics of the GMR elements under the influence of a weak magnetic field; however, this would not sufficiently contribute to the stability of the offsets.
Magnetized states that are unexpectedly moved under the influence of an intense magnetic field may be restored to the original ones by applying an initialization magnetic field to form thin film coils, which are embedded beneath the GMR elements. However, this method does not sufficiently contribute to the stability of the offsets.